Itaconic acid (IA), an essential compound used in manufacture of various products (e.g., acrylic fibers, rubbers, artificial diamonds, and lens), is highly demanded in the chemical industry. Certain microorganisms, such as Aspergillus terreus, produce this compound. It has been found that cis-aconitate decarboxylase (CAD) plays the key role in the biosynthesis of this compound.